1. FIELD OF THE INVENTION
This invention relates to a narrow band, relatively ultra stable radio apparatus for communicating alarm or data signals from a transmitter to a remotely located receiver, which receiver or transmitter may be carried on the person of individuals or which may be fixed to other devices with which they cooperate, such as smoke detectors, burglary sensors or paging devices, especially when the transmitter or receiver devices are battery operated and located within the same building.
2. PRIOR ART
The transmission of alarm or paging signals to alert individuals to the presence of fire, burglary or for other purposes comprises a segment of the communication art wherein brief messages must be communicated with high reliability. Radio systems in the prior art usually rely on subcarrier audio tones for coding purposes, and to achieve improved signal-to-noise ratios. These subcarrier tones modulate a radio frequency carrier, resulting in relatively wide band width radio signal transmission. For example, a multiplicity of subcarrier tones spaced over a few hundred cycles in separation are often used by prior art alarm devices to provide user code identification means; i.e., different paging receivers will respond to different combinations of these subcarrier tones. This ensemble of various combinations of subcarrier tones may encompass a wide radio frequency bandwidth of several thousand Hertz or more. Such a wide radio bandwidth may admit a substantial amount of interference from either intentional or unintentional signals and noise that may be on the same radio channel, or spurious signals spilling over from nearby channels. These interfering signals significantly reduce the desired signal-to-noise ratio when compared to the signal-to-noise ratio attainable in the instant invention. In the present invention the radio bandwidth is constrained to approximately 100 Hz or less, and in addition, the digital data stream logic pulses are also synchronized a priori in the transmitter and the receiver, and this provides a substantial improvement in system reliability.
With respect to prior patents, narrow band communication techniques are described in my earlier patents, such as U.S. Pat. Nos. 4,117,405, 4,208,630 and 4,415,771.